In what became the standard model of changes in stocks' prices, the statistical distribution of changes in the natural logarithms of stock prices is the normal distribution the canonical "bell-shaped" curve of statistical theory. This "lognormal" model is an example of what the mathematician and chaos theorist Benoit Mandelbrot calls "mild" randomness: the tails of the normal distribution, representing the probabilities of extreme events, are "thin." In the 1960s, Mandelbrot put forward a different model: one in which the tails are so "fat" that the standard statistical measure of a distribution's spread (the standard deviation or its square, the variance) is infinite.

Donald MacKenzie, An Engine, Not a Camera, loc. 138